Insulation without drywall?
Zone 4c
Using kraft faced fiberglass insulation, is it safe to insulate an exterior wall indefinitely without Sheetrock/latex paint? Eventually Sheetrock will go up, but wanted to keep the house warm for winter in the meantime…
I understand the drywall and latex paint to be a sort of vapor retarder, but I also know kraft paper has a perm rating of 1 which should mean it will keep mold at bay, right?
No rigid foam on the outside in this application, and there isnt any siding up quite yet but within a month or two there will be siding up on this wall and it will be vented. And currently there is tyvek housewrap.
To give this a little more context if it at all helps, I tried doing this same thing with another room but used un-faced mineral wool w/o Sheetrock for a while, and after only a few weeks I pulled out the batts to see what it looked like and sure enough, mold had started to grow. So trying this sort of thing without the Sheetrock makes me a little nervous, but hope my method this time around is smarter.
Any advice appreciated.
Thanks.
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Replies
I think you’re ok, but you could temporarily stable up a sheet of poly to act as an air barrier until you have drywall up. Just remove the poly before you put up the drywall. The other option would be to install a sheet-type smart vapor retarder like membrain over the wall which wouldn’t need to be removed before installing drywall.
The Kraft paper is a vapor retarder, but it’s almost never sealed very well. I wouldn’t rely on the Kraft facing alone, especially when there is no drywall to keep the edges pressed tight to the studs.
Bill
I could have mentioned this in my original post, but it seems more relevant now: I was planning to replace the kraft faced fiberglass insulation with mineral wool right before drywall/paint. Of course, I didn't want to put the fiberglass stuff in with the intention of taking it out later... perhaps there's a safe way to insulate w/ mineral wool instead of faced insulation for the several months before the drywall/latex paint stage. Here's my idea based on what you said:
You mentioned the membrain stuff... That got me thinking so I took a look in my garage and found a class II vapor retarding kraft paper normally used for beneath hardwood floors. Instead of the membrain, do you see any issue using this stuff on top of unfaced mineral wool for the foreseeable future w/o drywall and paint. Would this do well to stave off the mold? I could easily go with the membrain, I hear what you're saying, but if this stuff is just laying around I thought I could save a few bucks.
To me it seems this seems ok since i'd be up to code at that point - the code that says class I or II VR in marine 4.
If so, and if im on the right track - once I go to put up the drywall, and paint with a latex paint, is there any harm done by having 2 class II VRs in the same wall assembly? in other words, leave the paper up, or tear it down right before? (code in 4c says class III permitted w/ vented cladding.)
I don’t see any issues using the Kraft paper if it’s rated for the purpose. It won’t be much different from using paper faced batts. The most important thing for you in the temporary setup is to have something there to act as an air barrier before the drywall goes up.
Bill
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Probably a good reminder that airflow from the interior to the sheathing and then back to the interior can create a moisture problem. Interior side air barriers matter.
It will also help to keep interior dew point as low as possible.
Kevin,
Kraft-faced fiberglass isn't an air barrier, so you can still get mold on your sheathing. How moldy will your sheathing get? The answer depends on the outdoor temperature (colder temperatures = more mold) and the indoor relative humidity.
For more information, see this article: "Condensation on Wall Sheathing During Construction."
Martin, do you think the large sheets of Kraft paper would work as an air barrier, at least temporarily? My understanding has always been that Kraft faced batts aren’t an air barrier because the edges are never really sealed well so there are lots of gaps. If a large single sheet of Kraft paper was put up, almost like house wrap, I’d think that would act more like an air barrier if it was stalled tightly and/or caulked.
Admittedly I’ve never tried this, I’d normally just use poly sheet if I needed a temporary air barrier in an application like this.
Bill
You should also be aware that there is quite a bit of moisture built into a new home under construction. Concrete, lumber, everything that got wet/damp before the house was dried-in will now be trying to dry out, and all of that moisture in the air will be headed towards the cold sheathing. How are you heating the space? Let's hope you're not using a gas-fired "salamander" style heater. They add tons of moisture, not to mention toxic products of combustion.
If you want to minimize the risk of condensation and mold growth on the sheathing, you might want to dehumidify the interior air. For your conditions, dryer is better.
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Does any of this change if we're talking about a bathroom wall assembly with both outside and inside walls? Does a bathroom wall assembly need to be different for inside vs. outside walls? I'd like to use mineral wool in bathroom as well...
Is the use of VR paint or primer a good idea, or simply glossy latex paint over normal pva primer?
This bathroom will have an appropriately sized exhaust system.